The invention relates to a measurement device as well as a method for non-contact detection of oscillations of an object. The device of the type that includes a laser interferometer, a scanning device for generating a scanning movement, a data recording device, and an evaluation or display device.
Accordingly, the measurement device comprises at least one laser interferometer, at least one scanning device, which directs the measurement beam of the laser interferometer onto different measurement points of the object for generating a scanning motion, a data recording device, which interacts with the laser interferometer and with the scanning device in order to correlate the oscillation data measured by the laser interferometer with position data of each measurement point, as well as a display or evaluation device for position-matched display or evaluation of the oscillation data correlated with the position data.
Oscillations, especially vibrations, are conventionally measured by applying acceleration sensors onto the surface of the object and evaluating the signals of these sensors in correlation with the position data of the respective measurement points. However, very often such acceleration sensors cannot be used. For example, if very small structures, such as wires or write and read heads of hard disks, are to be measured, if the objects are very hot or very cold, or whenever the mass of the acceleration sensor produces inaccurate results for the oscillation mode of the object, particularly for lightweight objects and for soft structures, conventional sensors are not used. In addition, rotating objects, such as, e.g., hard disk drives, wheels of motor vehicles, or brake discs, cannot be measured with conventional acceleration sensors.
Therefore, optical methods for detecting oscillations of objects have been available for some time, because these methods enable measurements to be performed without mechanical contact with the object. Here, in particular, the optical measurement of the object by means of a laser interferometer offers not only an advantageously high measurement sensitivity, but also a wide bandwidth of the oscillation modes to be measured. Generally, a laser-Doppler vibrometer is used with a measurement point on the object being illuminated with coherent laser light. The oscillation movement of the object surface causes a Doppler shift in the frequency of the light reflected from the surface. An analysis of the frequency shift provides the desired oscillation data, because the respective velocity values can be calculated from the frequency shift. In addition, the acceleration values of the object at the measurement point can also be calculated from the progression of the velocity values. The principle setup of such a measurement device is described, e.g., in the German technical journal Technisches Messen—tm, 57 (1990), pp. 335–345.
A measurement device of the class mentioned above is known, e.g., from WO 93/15386. More specifically, this document describes how a scanning laser-Doppler vibrometer can be used for detecting the oscillation conditions of an object. The scanning device in this document is integrated in a measurement head of the interferometer and essentially consists of a moving deflection mirror for the laser beam. This deflection mirror helps to direct the laser beam in a scanning motion onto different measurement points of the object. This technology is widely used and is unproblematic in itself.
In many applications, it is necessary to detect the oscillation conditions of the individual measurement points not only in one dimension, that is, along the direction of the measurement beam, but also to record oscillation data for the measurement points in all three spatial directions. For such three-dimensional measurements, WO 93/15386 proposes to use three laser-Doppler vibrometers with measurement beams, which are aimed from different spatial directions onto the respective measurement points in order to be able to ultimately record three-dimensional oscillation data.
However, the deflection mirrors for the laser beams must be carefully synchronized, which is already rather complicated in itself. In addition, if the measurement points lie far apart from each other or even if a curved surface of the object is to be measured, then the focus for the individual measurement points must be changed continuously. Furthermore, it is not possible to measure at points that do not lie in the direct scanning region of the deflection mirror for the laser beams.